1. Field of Invention
The present invention relates to an inverter circuit and a method for supplying an inverted voltage; particularly, it relates to an inverter circuit and a method for supplying an inverted voltage, which have good driving capability to drive a power transistor. Such circuit and method for example can be used for supplying a negative voltage to a CCD (Charge Couple Device) circuit, etc.
2. Description of Related Art
An application of an inverter circuit is to provide a negative voltage to a CCD circuit. As shown in FIG. 1, in a prior art inverter circuit 1, a power switch driver control circuit 12 controls a power switch Q via a driver gate 14. By operation of the power switch Q and an inductor L, a voltage Vin from a battery BAT is converted to a negative voltage supplied to a CCD circuit, which requires a negative voltage such as −7V. The power switch driver control circuit 12 obtains a feedback signal from a feedback terminal FB and controls the gate of the power switch Q accordingly. Usually the power switch driver control circuit 12, the driver gate 14, the power switch Q and a 1.25 V voltage supply circuit are integrated within an integrated circuit 10.
The abovementioned prior art has the following problem. The high operation voltage and low operation voltage of the driver gate 14 are coupled to the voltage Vin supplied by a battery BAT and ground, respectively; that is, the gate of the power switch Q operates between the voltage Vin and 0V. Typically in CCD application, the battery BAT is two AA batteries supplying a voltage Vin of about 1.8V-3.2V. If the power switch Q is a PMOS, its threshold voltage Vt will be around −1 V. In other words, the circuit only has a driving capability of about 0.8V −2.2V to the gate of the power switch Q, which is too low, and the ON-time resistance of the power switch Q tends to change greatly according to the voltage of the battery BAT.
To solve the foregoing problem, the applicant has proposed the circuit as shown in FIG. 2. In this inverter circuit 2, a negative voltage charge pump 26 is provided in an integrated circuit 20, which switches the connections of the capacitors to generate a negative voltage CPO to be supplied as the low operation voltage for the driver gate 24. As a result, it can significantly improve both the gate driving capability to the power switch Q and the circuit stability. However, this method has the drawback that it requires capacitor devices external to the integrated circuit 20, and the integrated circuit 20 requires multiple additional pins. As shown in the drawing, three additional pins CP, CN, and CPO are required other than the original pins Vin, LX, FB and 1.25V. In consideration of cost, this is not ideal.
In view of the foregoing, the present invention provides an inverter circuit and a method for supplying an inverted voltage to solve the above mentioned problems.